Loss tangent is a parameter of a dielectric material that quantifies electromagnetic energy dissipation in the dielectric material. More specifically, the term refers to the angle in a complex plane between the resistive component of an electromagnetic field and its reactive component. For many applications, such as antenna design, printed circuit boards (PCB), hermetic packaging, etc, the electromagnetic energy can be considered as electromagnetic waves propagating either through free space, in a transmission line, in a waveguide or in a microstrip line. For mechanical or structural support of electrical conductors, dielectric materials are often used in these applications.
For example, in the case of an antenna system, because the electromagnetic wave travels through different parts of the antenna system (radio, feed line, antenna, free space, etc.), it may encounter differences in impedance. At each interface, depending on how well the impedance is matched, some portion of the wave's energy reflects back to the source of the wave, forming a standing wave in the feed line. Here, a standing wave is an electromagnetic wave that remains in a constant position, because of interference between two waves (one being the reflection of the other one) traveling in opposite directions. However, for waves having equal amplitudes and traveling in opposing directions, there is no net propagation of energy. Impedance matching deals with minimizing impedance differences at each interface to reduce ratio of maximum power to minimum power, that is, the standing wave ratio (SWR), and to maximize power transfer through each part of the antenna system.
Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, for example, by adjusting the length and width of the feed line. A Smith chart is a graphical chart to assist in solving problems with transmission lines and matching circuits. The Smith chart is used as a graphical indication of how many radio frequency (RF) parameters behave at different frequencies. It can also be used to represent many parameters including impedances, admittances, reflection coefficients, and Snn scattering parameters, among others.
There have been many attempts to measure or characterize the dielectric loss tangent of dielectric materials for different applications by complicated methods. These methods include filling a waveguide with the dielectric material of interest and measuring the radiation loss in the material. The measured radiation loss with the dielectric material is then compared with the measured radiation loss in the waveguide without the dielectric material to determine the loss due to the dielectric material of interest. Once the radiation loss to the dielectric material is determined, a loss tangent of the determined loss is calculated, for example, by a computer program. Other methods include resonant techniques such as waveguide resonators filled with the dielectric material, and resonant ring measurements.
However, these complicated methods are either not accurate and/or hard to implement, especially at higher frequency ranges. Resonant techniques typically measure insertion loss at very low signal resonant peaks. These methods are not adequate in measuring differences in loss involving small changes in dielectric material. At higher frequencies, measurement calibration inaccuracy limits the accuracy of the measurement for small signals levels. Therefore, a very limited frequency set is measured per structure with these conventional techniques. Furthermore, conventional techniques include filling waveguide with the dielectric material of interest and measuring antenna radiation loss. However, a coating is usually too thin to use in these bulk measurement techniques.
Therefore, there is a need for a less complicated and more accurate method for characterizing dielectric loss tangent of dielectric materials.